Magneto-Resistive Sensing Devices (MRSDs) are used to detect the presence of a magnetic field. Many of these magneto-resistive sensors are made of a material called Permalloy. Permalloy is a nickel-iron alloy. These sensors are used to detect the earth's magnetic field in compass applications. They are used to sense the proximity of a magnet, and are used in conjunction with magnets to determine rotational speeds of engines and other rotary devices. These sensors are also used to detect the stored information on disk drives of computers. Some applications of MRSDs allow a switch to be thrown by holding a magnet close to the sensor. Other applications allow a state to be set by non-contact means.
Many applications of MRSD are in portable and/or handheld devices. Such applications are often battery powered. Small device size and small device weight are both positive attributes of such instruments. Low power consumption allows such instruments to operate for long periods of time between battery charging and replacement. Furthermore, the smaller one can make the components of an instrument, the more functionality can be designed into the saved space or the instrument can be made to be smaller and lighter. Lower-power components can similarly be traded for either additional power consuming components or for longer battery life.